Recruitment of proteins to serve new functions has played a key role in the evolution of metabolic diversity during the nearly 4 billion-year history of life on Earth. This process continues to be important as bacteria respond to novel selection pressures imposed by human activities. Of particular importance to human health is the emergence of antibiotic resistance, which can arise by recruitment of a pre-existing protein to modify a newly introduced antibiotic. In addition, the recruitment of proteins to serve new functions in the biodegradation of toxic anthropogenic compounds is critical for removal of these toxins from the environment. Although it is clear that recruitment of proteins to serve new functions has occurred, the process has rarely been studied. This proposal describes studies of the process of recruitment of enzymes to serve a variety of functions that are critical to the survival of E. coli. The goal of this work is identification of genes in the E. coli genome for which over expression or mutation restores viability in mutant strains lacking critical metabolic enzymes by encoding proteins that can be recruited to serve the function of the missing enzyme. The following classes of enzymes will be targeted: 1) enzymes that catalyze very simple reactions, such as dehydration, oxidation of an alcohol, and phosphoryl transfer; 2) enzymes that have a common structural fold and for which many enzymes with a similar fold might be recruited to replace the missing enzyme; and 3) enzymes that have an uncommon structural fold, for which more creativity on the part of the bacterium may be required to find a replacement. Further studies will determine the level of the targeted enzyme activity in the recruited protein, as well as its original function and structural fold. For selected cases, in vitro evolution will be used to attempt to improve the level of the targeted enzyme activity in the recruited protein. The outcome of this work will be an expanded view of the process of recruitment and of the catalytic plasticity available in existing protein scaffolds. This work will provide insights into how Nature has exploited this plasticity to evolve new metabolic capabilities and will provide new information about the catalytic capabilities of certain protein scaffolds that can be exploited by protein engineers to create novel catalysts.